THE PETROLOGY OF RESERVOIR ROCKS AND ITS 
INFLUENCE ON THE ACCUMULATION OF 

PERTOLEUM. 


A. W.^fjAUER. 

Part of a Thesis Submitted to the Geological Faculty of Yale University in 
Partial Fulfilment for the Requirements o^ the Degree of Doctor of 

Philosophy 


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Economic Geology Publishing Company 


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[Reprinted from Economic Geology, VoI. XII., No. 5. August, 1917.] 


THE PETROLOGY OF RESERVOIR ROCKS AND iTS 
INFLUENCE ON THE ACCUMULATION OF 

PETROLEUM. 

A. W. Lauer. 


TABLE OF CONTENTS. 

Page. 

Introductory Summary . 436 

Review of Previous Theories . 436 

The Anticlinal Theory . 437 

The Hydraulic Theory. 438 

The Theory of Capillary Concentration. 435 

The Theory of Selective Segregation and Gravitational Separation .. 440 

The Filtration Theory . 441 

Openings in Rocks . 441 

Openings in Sedimentary Rocks . 442 

Table . 442 

Discussion of Table . 442 

(a) General Statement . 443 

(b) Original Openings . 443 

(c) Induced Openings . 443 

(d) Conclusions from Table . 445 

Openings in Sandstones. 446 

Table . 446 

Discussion of Table . 447 

Types Illustrative of the Various Classes of Porosity. 448 

(a) Original Openings . 448 

(b) Openings Chiefly Induced . 449 

(c) Original and Induced Openings . 451 

Openings in Limestones and Dolomites . 452 

Table . 452 

Discussion of Table . 453 

Illustrative Types . 453 

(a) Openings Original and Induced . 453 

(b) Openings Induced . 454 

The Trenton Limestone Fields . 454 

The Reservoir Rocks of the Oil Fields of Mexico 455 

Openings in Shales . 461 

Table . 461 

Discussion of Table . 461 

Illustrative Types . 462 

Conclusion . 462 

Acknowledgments . 464 

Plates XXIL-XXVIII. 466 

435 

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436 


A. W. LAVER. 


INTRODUCTORY SUMMARY. 

The recognition of petroleum in sedimentary rocks early led to 
the belief that it usually is not found where originally formed. 
The present “pools” indicate at least some migration from the 
beds of origin, with a subsequent accumulation in the rock for¬ 
mations where it now occurs. 

The features which control this migration have long been dis¬ 
puted among geologists, and in the opinion of the writer insuf¬ 
ficient attention has been paid to the part played by the porosity 
of the rocks. Consequently he undertook an investigation of 
rock openings in the hope that it might throw light upon this 
question. 

The study consisted of an examination, supplemented by micro¬ 
scopic evidence, of suites of specimens obtained from oil-bearing 
strata at variable depths in oil. wells widely scattered over the 
North American petroleum fields. Through the courtesy of the 
officials of the different oil companies, the geological surveys, and 
others, the writer has been singularly fortunate in obtaining 
specimens up to 9 inches in length, so that he feels considerable 
reliance may be placed on the data obtained from them. 

The investigation of the specimens has been supplemented by a 
broad study of the causes of rock openings of all kinds and sizes. 

The results reveal that porosity is not so much a matter of 
original interstitial openings, but that later induced openings have 
played a part hitherto insufficiently recognized. The result is to 
give more effectiveness to hydrostatic water action, and on these 
grounds plus a careful weighing of the merits of other theories 
proposed, the writer concludes for a defense of the anticlinal 
theory. 

REVIEW OF PREVIOUS THEORIES. 

To clarify the discussion and to present a summary of present 
views held by oil geologists, the theories of oil movement and 
accumulation now in the literature will be briefly reviewed. 
These include the anticlinal theory, the hydraulic theory, the 
theory of capillary concentration, the theory o'f selective segrega¬ 
tion and gravitational separation, and the filtration theory. The 

« 

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free 


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DEC KT? 


PETROLOGY OF RESERVOIR ROCKS. 


437 


first four have grown out of attempts to reconcile the facts of oil 
occurrence to physical principles governing the movement of 
liquids through rock material while the fifth is developed from a 
slightly different viewpoint. 

The Anticlinal Theory. 

Probably the best statement of this theory in its present form, 
somewhat modified from the original developed by Orton, White, 
and others, is that by Griswold and Munn d 

‘‘ Whether the petroleum comes from within or from below the shales, 
it must pass through them, and to do this it must pass through the very 
small pores existing in those relatively impervious beds. The nature and 
causre of this movement are not understood. Capillary action and great 
rock pressure may be suggested as causes which aid in forcing the 
petroleum out from the shales, but there are not sufficient data on this 
subject to justify any scientific explanation. It matters little what is 
the ultimate source of the oil; the important facts are its occurrence 
now in the porous sandstones, its circulation through the rocks, and the 
conditions leading to its accumulation in commercial deposits.” 

‘‘Movement in Porous Rocks. The porous rocks into which oil and 
gas enter may be dry, or they may be completely saturated with water. 
In most cases it is probable that a combination of these two causes exist— 
that the porous rocks are completely saturated with water up to a certain 
level, but above that point they are dry. The movement of the hydro¬ 
carbons through the rocks will not be the same in the two cases, and 
therefore each condition must be considered separately.” 

“ If small quantities of oil and gas enter a dry porous rock at different 
points the oil will flow down as long as gravity is sufficient to overcome 
the friction and the capillary attraction. The gas will diffuse with the 
air or water-vapor contained in the pores of the rock.” 

“ Oil and gas entering a porous rock that is completely saturated with 
water will be forced up to the top of the porous stratum by the difference 
in the specific gravity of the hydrocarbons and the water. Here the oil 
and gas will remain if the porous stratum be perfectly level, but if it has 
a dip sufficient to overcome the friction the particles of oil and gas will 
gradually move up this slope, the gas with its lower specific gravity 
occupying the higher places.” 

“ In case the porous rocks are partly saturated a combination of these 

1 W. T, Griswold and M. J. Munn, “ Geology of Oil and Gas Fields in Steu¬ 
benville, Burgettstown, and Claysville Quadrangles, Ohio, W. Va., and Pa.,” 
U. S. G. S. Bull. No. 318, 1907, pp. 13-14- 


438 


A. IV. LAUER. 


two actions will take place. The oil entering above the line oi complete 
saturation will flow down to that line and the oil entering below will be 
forced up to the top of the completely saturated portion.” 

“ The statements given above are based on the assumption that the 
oil-bearing rock is homogeneous throughout and that the oil will move 
with the same degree of freedom in every direction. This is rarely the 
case. Sandstones are noted for their irregularity in composition, as 
regards both the size of the individual grains of sand and also the 
material which cements the grains together. It is obvious that any fluid 
will move more rapidly through a coarse conglomerate imperfectly 
cemented than through a dense, fine-grained sandstone the particles of 
which are thoroughly coated and all the interstices filled with impervious 
cement. If the oil-bearing rock contains areas practically impervious, 
these areas, according to their size and position, will be more or less 
perfect barriers against the movement of the oil or the gas.” 

To this theory some objections were later raised by Munn^ on 
the basis of observations in the oil pools of the Sewickley cjuad- 
rangle, Pennsylvania. It was found that there was ‘‘no per¬ 
ceptible difference in the ratio of water to oil in the pay streaks at 
the top and bottom of the sand,” and that “a lower pay streak 
may frequently be found to furnish a greater per cent, of oil to 
water than another located above.” It was also found in one 
case that in a portion of the sand the salt water gained headway 
up the dip as the oil was exhausted through producing wells. So 
far this corresponded to the anticlinal theory. Down the dip of 
the same producing sand, however, a distance of less than three 
miles from these wells, this sand was found to be dry. To ex¬ 
plain these and similar facts Munn developed 

The Hydraulic Theory.'^ 

This theory makes the accumulation of oil and gas dependent 
on moving water, water traveling through the rocks by hydraulic 
(“not hydrostatic”) pressure and by capillarity, forcing the par¬ 
ticles and small globules of oil before it, and allowing them to 
collect in pools where two or more such moving masses of water 

2 Malcolm J. Munn, “ Studies in the Application of the Anticlinal Theory 
of Oil and Gas Accumulation,” Econ, Geol., Vol. IV., 1909, pp. 141-157. 

3 M. J. Munn, “ The Anticlinal and Hydraulic Theories of Oil and Gas 
Accumulation,” Econ, Geol., Vol. IV., 1909, pp. 509-529. 


PETROLOGY OF RESERVOIR ROCKS. 


439 


conflict, or form an eddy. Capillarity is also thought to operate 
in segregating much oil into the more porous rocks. The accumu¬ 
lation and concentration process is looked upon as probably tak¬ 
ing place subsequent to the desaturation of the original oil and 
gas-bearing beds. This process is described as occurring by 
means of the pressure of the overlying accumulating sediments 
while still on the sea bottom. According to the theory also, “ the 
pools of oil and gas are held in place by water under hydraulic 
and capillary pressure, which effectively seals up all the pores of 
the surrounding rock and prevents the dissipation of pressure 
by diffusion.” 

Pressure in pools is maintained by the expansive force exerted by 
gas, which was either accumulated with the oil or generated subsequent 
to the formation of the pools. Such gas could not diffuse because of the 
saturated condition of the surrounding rocks. 

To the exceptional structure in the Sewickley quadrangle, upon 
which this theory is based, Clapps shows that the evidence when 
broadly considered is not “decidedly against the idea that the 
pools could have been accumulated by difference in gravity of oil 
and salt water.” 

Whatever may be the local effects of water acting in a purely 
hydraulic manner, such action is subordinate to the general hydro¬ 
static flow of underground water, which moves from topograph¬ 
ically higher intakes, to points of lower elevation, or to issue as 
springs lower down. 

The Theory of Capillary Concentration. 

This has been proposed by Washburne^ as a general explana¬ 
tion of oil accumulation, with especial reference to one widely 
observed fact, namely the occurrence of oil in the more porous 
parts of a sand reservoir. In preference to other portions. 

He points out the relative surface tensions of water and oil, 
which are about 75 and 25 dynes per centimeter respectively, or 

4 Frederick G. Clapp, Discussion of Mr, Munn’s theory, Econ. Geol., Vol. 
IV., 1909, pp. 565-570. 

5 Chester W. Washburne, “The Capillary Concentration of Gas and Oil,'’ 
Trans. A. I. M. E., Vol. 50, 1914, pp. 829-842. Discussion, 842-858. 


440 


A. W. LAUER. 


as 3 to I. Capillary openings range from .508 mm. diameter for 
tubular ones, down to .0002 mm.; and for sheet spaces from those 
with a width of .254 mm. down to as small as .0001 mm. 

Within this range capillary action operates, and since its power 
increases’as the surface tension and inversely as the size of the 
opening, water would enter the rock pores of the smallest dimen¬ 
sions, in advance of, and displacing oil, which would be forced 
into the remaining coarser spaces. 

A working hypothesis in partial harmony with the anticlinal 
theory is suggested by Washburne along these lines: Gas would 
seek the highest points of arches and. domes through its great 
mobility and low specific gravity. Oil would collect for the most 
part in the coarser openings by capillary action, and thereafter 
masses of it would often be carried by rising water up to a point 
beneath the gas or to the water-gas surface, where it would there¬ 
after be held by surface tension. Some oil would also probably 
be carried up by gas as envelopes of bubbles, and come to rest at 
the water-gas surface, collected and held together by surface 
tension. 

The Theory of Selective Segregation and Gravitational 

Separation. 

Johnson and Huntley® have explained oil movement as due to 
“selective segregation” brought about by capillarity, immisci- 
bility, and relative viscosity of oil, gas, and water. The agency 
of gravity is considered active only when not subject to capillary 
interference, and when movement takes place. 

The efficacy of the latter factor is probably very great, and so 
far as observed by the writer this statement is generally admitted. 
It not only appeals as being reasonable, but has been demonstrated 
experimentally by Washburne and others. In the writer’s opinion 
either crustal movement or movement of water in the rocks would 
tend to counteract the retention of oil in coarser capillary spaces 
after a certain quantity had accumulated. The oil would then 
likely be carried upward to the water-gas surface. 

® Roswell H. Johnson and L. G. Huntley, “ Principles of Oil and Gas Pro¬ 
duction,” first edition, 1916, pp. 48-49. 


PETROLOGY OF RESERVOIR ROCKS. 


441 


The Filtration Theory. 

Dr. Day and others found that when clay or fuller’s earth is 
allowed to take up oil, fractionation occurs, and the oil driven out 
later when the clay is treated with water, is not the same as the 
oil which was first taken up by the clay. It was found that the 
oil removed had a much lower percentage of unsaturated hydro¬ 
carbons. 

On these findings the theory is suggested that in those fields'' 
where the oil has traversed dry clays and shales in reaching its< 
final “pool,” or place of accumulation, considerable alteration by 
fractionation must have occurred. This would be one explana¬ 
tion of why oil varies so, sometimes in short distances. 

OPENINGS IN ROCKS. 

The hydraulic and capillary theories indicate that an over¬ 
emphasis has been placed on rock pores and openings of capillary 
size, thus making the movement of oil dependent on the move¬ 
ment of water and on differences in surface tension, with a very 
small role to be played by gravity. 

To understand openings in rocks, and their relative sizes, it fs' 
advisable to consider, not only as interstitial spaces, but also from 
a broader view taking into consideration the effects of diastrophic 
movements, stratigraphic relationships, and the incomplete cemen¬ 
tation of sedimentary beds. 

The question of rock openings has been most thoroughly 
treated by economic geologists, and the following table was ar¬ 
ranged after assembling all known and possible openings in sedi¬ 
mentary rocks, of whatever cause or size, which are referred tO' 
in the writings of Van Hise,^ Lindgren,® and Irving.^ These 
were supplemented by others as hereinafter explained. 

Chas. R. Van Hise, “ Some Principles Controlling the Deposition of 
Ores,” Trans. A. 1 . M. E., Vol. 30, igoo, pp. 38-39- 

8 Waldemar Lindgren, “ Mineral Deposits,” first edition, 1913, Chap. 10. 

9 J. D. Irving, Lectures in Economic Geology, Sheffield Scientific School,, 
Yale University. 


442 


A. IV. LAUER. 


(1) 

( 2 ) 

( 3 ) 

( 4 ) 

(I) 


( 2 ) 

( 3 ) 

( 4 ) 

< 5 ) 

< 6 ) 

( 7 ) 


Openings in Sedimentary Rocks. 

Super-capillary ( >.508— .'’54 Mm.). Capillary (< .508— .254 Mm.). 


Original. 

Drying cracks (mud cracked rock 
beds). 

Rock pores (conglomerates, some 
limestones). 

Shell cavities (some shell lime¬ 
stones). 

Dolomite cavities. 

Induced. 

Due to folding (fissures, radial 
cracks, bedding partings, synclinal 
slumping). 

Compressional brecciation and con¬ 
jugate fissures. 

Dolomitization, cavities. 

Fissuring and rupturing apart from 
folding (joints, fissures by tor¬ 
sional stress). 

Solution cavities in limestone, dolo¬ 
mite, gypsum, and salt. With these 
belong also 

Fossil cavities dissolved out. 

Effects of former surface weather¬ 
ing below unconformities. 


(1) Drying cracks. 

(2) Rockpores(sandstones,shales, 

limestones). 

( 3 ) -- 

(4) Dolomite pores. 

(1) Due to folding (fissure open¬ 

ings, radial cracks, bedding 
partings, synclinal slump¬ 
ing). 

(2) Compression brecciation and 

conjugate fissures. 

(3) Dolomitization pores. 

(4) Rupturing apart from fold¬ 

ing (joints and small tor¬ 
sional fissures). 

( 5 ) - 

( 6 ) - 

(7) Effects of former surface 
weathering below uncon¬ 
formities. 


DISCUSSION OF TABLE. 

{a) General Statement. 

The openings listed above are divided into original and induced 
on the basis of cause, and into capillary and supercapillary on the 
basis of size. Subcapillary openings are omitted because they do 
not serve as channels of underground movement of either water 
or oil. The dimensions for capillary openings are those quoted 
by Van Hise and others. The table is largely self-explanatory, 
but some of the openings should be called to the reader’s atten¬ 
tion. All the known possible openings are listed but a few may 
be eliminated because experience does not indicate that they are 
generally developed. These will be referred to in their proper 





PETROLOGY OF RESERVOIR ROCKS. 


443 


order. Some openings are placed only in the supercapillary list 
because that is their characteristic development. This does not 
deny the possible existence in isolated cases of related openings 
which attain to only capillary size. 

(b) Original Openings. 

Drying cracks are usually entirely refilled, hence in practice 
would not often be found to exist. 

The rock pores of some conglomerates are exceedingly coarse 
and may be supercapillary. In the case of limestones such as 
those of “ Corniferous ” age in the Irvine, Kentucky oil field, the 
writer has samples ranging from very dense close-grained rock, 
to spongy, open, cellular rock whose pores are clearly super¬ 
capillary, and form connecting spaces of considerable volume 
(see Plates XXIIL, A and XXII., D). 

Conspicuous openings of an original type are found also in some 
of the coquinas. These are referred to as No. 3 in the table. 

(c) Induced Openings. 

Under No. i of the later, or induced openings in rocks, those 
due to folding include a great number whose size and variety were 
first appreciated in the Wisconsin-Iowa-Illinois lead and zinc 
mines. These occur as important vein-filled channels cutting 
across the beds. There are also flat openings, or flats,” parallel 
to bedding planes and containing ore. Both types range in size 
from mere seams to several feet in width. 

Radial cracks hardly appear in gentle folds, except as incipient 
openings. In close folds they may open and considerable dis¬ 
turbance result at crests and troughs because of their presence. 

As pointed out by Lindgren,^® large important openings may 
also occur at the tops of anticlinal arches by a parting and slipping 
along bedding planes, and are most important in the case of rock 
beds of unequal resistance. This class of openings may merge 
into capillary-sized spaces, but their development is generally 
larger. 

10 Op. cit., p. 135. 


444 


A. W. LAUER. 


Openings caused by compression, crushing, and the develop¬ 
ment of conjugate and complex fissure systems are deemed of 
great importance, and deserve especial mention in cases where 
they have developed fracture zones and belts of mashing. 

Dolomitization cavities are important in many rocks as in the 
Trenton limestone, and in the dolomitic lenses which form the oil 
horizon for certain of the coastal dome pools. 

Such openings are listed in all four parts of the table. This 
is done advisedly because of the latest research on the question of 
dolomitization, also because even the recent views are not yet 
harmonized. Thus some paleontologists find clear evidence that 
dolomitic replacement and alteration go on while deposition is in 
progress. Schuchert^^ favors the opinion that this has occurred 
in many cases where widespread dolomites occur, and that by 
diagenetic changes a porous dolomite has resulted. In such in¬ 
stances dolomite pores and cavities cannot be called induced, or 
subsequent openings. 

On the other hand the openings of some dolomites are best ex¬ 
plained as resulting from atmospheric leaching long after their 
formation,^ 2 and both this type of vesicularity and shrinkage due 
to replacement of lime by magnesia may result in producing 
openings. Thus dolomitic cavities are to be variously explained 
and appear to belong to both original and induced openings. 

Fracturing, and rupturing apart from folding, may produce 
communicating channels of great extent in sedimentary strata. 
If they occur as joints they are frequently no larger than capil¬ 
lary openings, but where faulting has taken place they may as¬ 
sume dimensions of strictly supercapillary importance. The same 
is true where fissures develop caused by torsional stresses, and, 
due to tension, these may open to considerable width. 

There also occur in limestones, dolomites, gypsum, and salt 
bodies solution cavities, which occasionally assume dimensions 
of extraordinary size. In so far as solution can be effective, all 
cavities, in rocks of whatever kind, may be enlarged, and the 

Chas. Schuchert', personal communication. 

12 Francis M. Van Tiiyl, “ New Points on the Origin of Dolomite,” Amer. 
Jour, of Sc., Vol.. XLIL, 1916, p. 256. 


PETROLOGY OF RESERVOIR ROCKS. 


445 


action of underground circulation may thus contribute in giving 
to many originally capillary openings a supercapillary size. 

Item numbered 6 , fossil cavities dissolved out, will be seen to 
be of considerable importance in the Mexican oil reservoirs. 

The final class of openings, surface weathering below uncon¬ 
formities, is one not referred to in literature on rock openings 
as such. They are concerned with observations in stratigraphy 
rather than economic geology, and the writer is indebted to 
Schucherfi^ for having his attention called to them. 

Where formations occur bounded above by surfaces of uncon¬ 
formity or disconformity, they are often found to be exceed¬ 
ingly Open and porous. Their porosity is ascribed in large part 
to the rotting, weathering, and general disintegration processes 
which they suffered during the interval of erosion when they 
were exposed as a land surface. While resubmergence was doubt¬ 
less accompanied by saturation and partial closure by recementa¬ 
tion and infiltration of matter, it is thought that much of their 
present porosity is due to the effects of the disintegration for¬ 
merly suffered. 

(d) Conclusions from the Table. 

The table thus gives a better conception of «the different classes 
of openings through which underground waters or other liquids 
circulate. 

As Van Hise^^ has pointed out, all movement of a capillary 
nature is exceedingly slow, while through supercapillary openings 
it follows the ordinary laws of hydrostatics. Lindgren^^ modifies 
this slightly by saying it “is subject to a certain retardation on 
account of friction.” Furthermore, the supercapillaries are not 
confined to any one bed, but may connect a series of beds ver¬ 
tically and laterally (the latter by bedding partings which form 
important channels for sidewise circulation, and for connecting 
vertical openings which offset one another). They are thus the 
trunk channels, and .the capillary openings of all kinds, whether 

13 Chas. Schuchert, personal communication. 

14 Op. cit., p. 41. 

15 Op. cit., p. 32.. 


446 


A. W. LAUER. 


as original pores or as induced joint spaces, perform their func 
tion as subsidiaries to the main channels. The practical recog¬ 
nition of this is seen in those oil fields where the ‘‘shooting” of 
wells with nitroglycerine is carried on. In many wells the oil 
“ sand ” is encountered at places lacking .the trunk channels, and 
oil seeps into the hole very slowly through capillary pores. By 
“ shooting,” trunk channels are induced which radiate from the 
well-bore in all directions, connect with other channels, and thus 
divert a large flow to the well. 

To determine more closely the special forms of openings pecu¬ 
liar to the several kinds of oil horizons, and thus to enable the 
practical application of the discussion to be readily pointed out, 
it is advisable to subdivide the table of rock openings on the basis 
of the classes of oil horizons. Each of these classes may then be 
considered separately, with all the geologic factors which have 
contributed to its present state as a porous oil reservoir. This 
method also makes it possible to credit with proper importance 
the reservoirs other than sandstones, as the latter have been given 
a position perhaps more prominent than they deserve. 


OPENINGS IN SANDSTONES 16 


Super-capillary O. 508-.254 Mm.) 


Capillary ( <. 508 -. 2.^4 Mm.) 


ORIGINAL 


(1) 

Rock pores (interstitial spaces). 

INDUCED 

(1 ) 

(1) 

Due to folding (fissures, bedding 
partings, synclinal slumping). 

(1 ) 

(2) 

Compressional brecciation and con¬ 
jugate fissures. 

(2.) 

i 2 ) 

Fissuring and rupturing apart from 
folding (joints and fissures by tor- ‘ 


sional and tangential stresses). 

(3 ) 

(4 ) 

Solution cavities in calcareous 

sandstones (?). 

(.' ) 

Effects of former surface weatner- 
ing below unconformities. 




(4 ) 


(r>) 


Rock pores/ (intersti¬ 
tial spaces). 

Due to folding (fis¬ 
sures, bedding part¬ 
ings, synclinal slump¬ 
ing) . 

Compressional breccia- 
tion and conjugate 
fissures. 

Fissuring and ruptur¬ 
ing apart from fold¬ 
ing (joints and fis- 
ures by t^ors’onal 
stresses). 

Solution cavities in 
calcareous sandstones 
(?). 

Effects of former sur¬ 
face weathering be¬ 
low unconformities. 





PETROLOGY OF RESERVOIR ROCKS. 


447 


DISCUSSION OF TABLE. 

In the general table on page 442 the original openings include 
fossil cavities. While these may occur sparingly in sandstones, 
they are too unimportant for mention. The spaces between sand 
grains are thus the most important original pores, and, except 
in some coarse conglomerates, are of capillary size. 

Of -the induced openings, the magnitude will depend upon the 
following factors: First, upon the amount of deformative strain 
the region has suffered; second, upon the thickness of the sand¬ 
stone beds, and their relation to, and the character of, the inter¬ 
vening strata, that is, whether the latter may transmit a fracture or 
opening into underlying or overlying sandstones, or whether they 
are merely soft shales which quickly absorb the forces affecting the 
harder rocks. Finally the size of openings induced will depend 
upon the resistance offered by the sandstones themselves, a factor 
governed by the strength and nature of the cementing matrix. 
If the latter is weak or scarce, the sandstone is only a loosely con¬ 
solidated sand which yields to movement by crushing. If 
strongly cemented, the sandstone will yield by broader folding, 
or be definitely fractured, jointed, or broken. If partially 
cemented, or only in certain areas, intermediate results will follow. 

In the first case, obviously interstitial spaces represent the 
dominant form of porosity. In the second case, the interstitial 
space may become almost negligible, and cracks, joints, and 
fissures become the dominant form of porosity. In the third 
case a combination of original and induced porosity results. 

The presence of all these factors of lithology, deformation, 
and stratigraphy, all variables, makes it evident that the pore 
space effective in controlling the movement and accumulation of 
petroleum, becomes a question of considerable perplexity. This 
suggests the futility of seeking accurate estimates of future pro¬ 
duction based upon determinations of the average per cent, 
porosity (original) in well samples from a partially explored oil 
sand. It is obvious also that pore space derived from samples 
of an oil sand give no clue whatsoever as to how much open space 
exists in the parent rock in the form of induced openings. 


448 


A. W. LAUER. 


While inaccurate, such investigations of future production may 
be of some value in aiding the “ feeling out” process in develop¬ 
ing a field, for the purpose of ascertaining as far as possible the 
several factors which contribute to its porosity. 

TYPES ILLUSTRATIVE OF THE VARIOUS CLASSES OF POROSITY. 

(a) Original Openings. 

A sample of the Hoing sand, the productive horizon of the 
western oil fields of Illinois, may be considered typical of this 
class. This sand, from the Colmar field, is a light brown, rather 
fine-grained, porous, quartz sandstone with an indistinguishable 
cement. The grains are in part sharp, and it appears open tex¬ 
tured in the hand specimen, yet does not crumble readily. Its 
open nature would give it chiefly an original, or interstitial poros¬ 
ity (see Plate XXIL, A). 

Similarly, a specimen from Red Fork, Oklahoma, from the 
Taneha sand of Pennsylvanian age, is made up of clean, light-’ 
colored, fine-grained quartz very poorly cemented'. As a result 
the rock can be crumbled with ease between the fingers. 

Of a similar character is the oil sandstone from the “ Tiger 
Flats.” 

The most friable Oklahoma specimen, however, is one from 
the Cushing field, a pale tan sandstone, made up principally of 
sharp or subangular grains of quartz with a slight admixture of 
arkosic material, and some scattered fragments of calcite. A 
photograph of some of the sand grains rubbed from this fragile 
rock appears in Plate XXIL, C, while R is a photograph of the 
specimen itself. 

A still more friable specimen comes from the Santa Maria, 
California field. It is a fine-grained arkose, though not uniform, 
and shows in thin section a number of considerably altered feld- 

In the description of rock types the writer makes use of specimens se¬ 
cured from widely scattered sources. Especial attention has been paid to the 
descriptive data accompanying each sample, and of those submitted here and 
in later portions of the paper, it is felt that their origin and genuineness is 
unquestioned. The validity of the data is therefore to be accepted, and the 
material can properly be presented for study. 


PETROLOGY OF RESERVOIR ROCKS. ' 


449 


spars. Quartz is present in grains of all sizes within the textural 
range of the rock. 

In this class of loosely consolidated sands, tests of interstitial 
porosity should give the closest approximations as to saturation 
of sands. Yet in California, where this type of reservoir pre¬ 
dominates, the results of such tests have not always been satis¬ 
factory. Pepperburg^® cites instances where, by carefully follow¬ 
ing rules worked out by the geological survey for estimating the 
probable oil contained in an undrilled, undeveloped district, one 
is very much disappointed in the results obtained. In one case 
where 60,000 barrels per acre were expected, production proved 
to be 250,000 barrels per acre. In others, where the estimate 
was 100,00 barrels per acre, production data extending over sev¬ 
eral months and even years, proved far short of this amount. 

The collection of rocks from which the foregoing samples have 
been selected and described were gathered by the writer largely 
for the purpose of making quantitative porosity tests. But when 
the problem was analyzed it became evident that such investi¬ 
gations could not yield dependable results, and the best that can 
be done seems to be to outline the factors involved as has been 
attempted. 

(b) Openings Chiefly Induced. 

The writer's collection includes a piece of Berea sandstone 
from the Corning oil '‘pool” of Perry County, Ohio, which was 
shot from, a well at a depth of 1120 feet. This rock in the hand 
specimen is a dense, even-grained, very uniform, nearly white 
sandstone. Viewed in thin section, it is an aggregate of pure 
quartz grains set in a solid matrix of calcite. Estimated by the 
eye alone, the calcareous cement occupies about 50 per cent, of 
the area of the section. The quartz grains are sharp to sub- 
angular, and the rock appears to be so closely cemented that it is 
difficult 'to see how either water or oil could penetrate. The grip 
which the cementing bond of calcite has upon this rock, and the 
thoroughness of its penetration and interfilling is forcibly brought 
to attention by the way whole areas extinguish at once between 

Leon J. Pepperburg, communication to the writer. 


450 


A. W. LAUER. 


crossed nicols. This indicates a strong tendency to crystallize 
in large crystals in spite of the presence of the quartz grains. It 
is difficult to see how oil in commercial quantities could be stored 
in such a rock, or if present, how it would yield itself fast enough 
upon being tapped to furnish a paying supply, because such a 
capillary flow would be very slow. In those ‘‘ pools ” of Ohio 
where rich strikes have been encountered in this rock it must 
either have had a much weaker binder, as Bownocker^^ claims 
generally for it, or the porosity would have to be looked for 
largely in the openings readily induced by any stress tending to 
deform so rigid a rock. These would be prominently developed 
at the crests of folded areas and would provide upward passages 
from the bituminous shales which underlie it, and are looked upon 
as the beds of origin. 

Another sample, from the Bald Hill “pool” of Oklahoma, is 
tight and well cemented. It consists of a fine-grained aggregate 
of quartz grains, not very sharp, and interlocking in exceptionally 
close contact throughout. In such a reservoir one should expect 
to find an appreciable percentage of the effective porosity in 
joints, cracks, and other induced openings. Such seems to have 
been the case as Hutchison^^ found that the limited surface struc¬ 
ture in the Bald Hill-Morris district indicated close crumpling 
and mashing. The somewhat erratic nature of production as¬ 
cribed to the district lends support to the same conclusion. 

Several samples secured from districts in southern California 
are very close, tight rocks. They are from oil-bearing horizons, 
and while unusual because of the generally loose and uncon¬ 
solidated character of the California oil sands, they are said to 
be representative of the strata from which they come. 

One of these is a medium-grained, dense arkose-conglomerate 
from the Fullerton oil fields. 

A second from the Salt Lake field has fine quartz grains 

J. A. Bownocker, “Oil and Gas,” Geol. Survey of Ohio, Bull, i, 4th ser., 
1903, p. 22. 

20 L. L. Hutchison, “ Preliminary Kept, on the Rock Asphalt, Asphaltite, 
Petroleum, and Natural Gas in Oklahoma,” Okla. Geol. Survey Bull. No. 2, 
1911, pp. 216-220. 


PETROLOGY OF RESERVOIR ROCKS. 


451 


cemented with calcite, the whole containing larger darker masses 
of some hard much altered unrecognized material. 

A third from the Maricopa field is chiefly arkosic and the 
grains are bedded in a calcareous cement. 

A fourth is very fine-grained and not so closely cemented. It 
is a gas sand from the Midway fields and consists of an arkose 
with a matrix of calcite. 

The density of these rocks precludes that porosity restricted to 
original interstitial spaces would yield a big production, especially 
in the heavy oils of California. The development of complex 
structures undergone by the rocks of this region would produce 
in such hard strata numerous induced openings. To such again 
it seems not unreasonable to look for an important source of 
their porosity. 


(c) Original and Induced Openings. 

There remain for consideration the intermediate types of sand' 
stones which arise from poorly assorted sediments, such as argil¬ 
laceous sandstones, or those in which the cementing bond is so 
variable in amount and character that no similarity exists in the 
samples from well to well of the same “ pool.” In this class alsa 
may be placed for convenience such rocks which may be pure,, 
but are neither rigid nor unconsolidated. 

The argillaceous type appears to be represented by one small’, 
sample from the Lander, Wyoming, field. It is made up of very 
fine grains of quartz, some calcite, and a large proportion of clay. 
It is friable enough to be placed in the class of loosely consoli¬ 
dated rocks, but the presence of so much clay would be likely to 
render it impervious in some parts of the “ pool,” hence variable- 
in its oil content. In the same class belongs the Booch sand ”’ 
from the Bald Hill-Morris district in Oklahoma, whose micro¬ 
scopic character and shreds of gray shale visible to the naked eye 
indicate it is a rather impure sandstone. 

The rocks of variable cementing bond are well illustrated by 
specimens shot from the sandstone reservoir of the Broken Ar¬ 
row, Oklahoma field, which appear to be hard but can be rather 
easily broken by hand. In color they are a clean light yellow. In 


452 


A. W. LAUER. 


thin section some areas are merely a closely packed series of half- 
rounded and subangular grains of quartz, while in others the in¬ 
terstitial spaces are filled with calcite. The same type of variably 
cemented rock, but in a less pure sandstone, is illustrated by a 
specimen from the eastern edge of the Bird Creek, Oklahoma, 
pool,” in the salt water area. This sample is from the Bartles¬ 
ville sand. 

Rocks of the intermediate type may be illustrated by one from 
the Muskogee oil sand, of basal Pennsylvanian age, and of a clear 
grayish-white color. Examined in thin section, it is seen to con¬ 
sist of fine and very fine quartz grains, closely spaced, or other¬ 
wise. 

The individual samples of all three classes mentioned above are 
merely illustrative, and determinations made for any field should 
include as many samples as possible in order to obtain represen¬ 
tative results. 


OPBNIN'GS IN LIMESTONES AND DOLOMITES. 21 


Super-capillary ( > .508-.254 Mm.) 

ORIGINAL 

(1 ) Rock pores in limestone and dolo¬ 
mite. 

(2 ) Fossil cavities in shell limestone, or 
coquina. 

(2 ) Dolomite cavities. 

INDUCED 

(1 ) Due to folding (fissures, * bedd ng 
partings, synclinal slumping). 

(2) Compressional brecciation and con- 

.iugate fissures. 

(3 ) Dolomitization cavities. 

(4 ) Fissuring and rupturing apart from 

folding (joints, fissures by torsional 
stress.) 

(5) Solution cavities in limestone and 
dolomite, including 

(6 ) Fossil cavities dissolved out. 

(7 ) Effects of former surface weather¬ 
ing below unconformities. 


Capillai'y ( <.508-.254 Mm.) 


(1 ) Rock pores in lime¬ 
stone and dolomite. 

(2 ) - 

( 3 ) Dolomite pores. 

(1 ) Due to folding (fis¬ 
sures, bedding part¬ 
ings, synclinal slump¬ 
ing). 

(2 ) Compressional brec¬ 
ciation and conjugate 
fissures. 

(3 ) Dolomitization pores. 

(4 ) Fissuring and ruptur¬ 
ing apart from folding 
(joints, fissures by 
torsional stress.) 

(5) - 

(6 ) - 

(7 ) Effects of former sur¬ 
face weathering be¬ 
low unconformities. • 






PETROLOGY OF RESERVOIR ROCKS. 


453 


DISCUSSION OF TABLE. 

Since limestones, magnesian limestones, and dolomites are 
essentially rigid rocks, it follows that the classification of their 
pores is not dependent, to the same extent as in sandstones, upon 
a varying degree of resistance offered to deformative stress. 

Where dolomitic reservoirs are treated in the following discus¬ 
sion, no attempt is made to classify that part of their porosity 
due to dolomitization as induced or original, because the criteria 
are not at hand to determine which it is. 

ILLUSTRATIVE TYPES. 

(a) openings, Original and Induced. 

Munn^^ found the Beaver Creek oil ‘‘ sand,” in Wayne County, 
Kentucky, to contain important natural pores between interlock¬ 
ing calcite crystals, and this rock serves as one example of the 
first class of oil-bearing limestones. It is illustrated in Plate 
XXII., D. For want of more data it is assumed by the writer 
that this porosity is the dominant variety. As the rock, however, 
is described to be a “ cherty geode-bearing limestone,” it seems 
not unlikely that openings induced by former traveling solutions 
and secretionary processes may contribute to the original pore 
space. Finally openings probably exist induced by folding, and 
in joints. 

Other specimens from Kentucky are shown in Plate XXIII., 
A, which constitute the Irvine oil “ sand,” of the Irvine field in 
Estill county. 

The porosity of these pieces, as well as that of the Beaver 
Creek, is noticeably variable, and in the case of the Irvine field, 
Glenn^s states that the wells which strike the “ loose sand ” rarely 
require shooting, while those in the “ tight sand ” dO'. 

A type probably containing much more effective original poros¬ 
ity than either of the preceding limestones, is the McCloskey 
^^sand” from the upper part of the Ste. Genevieve (Upper 
Mississippian) formation in Illinois (see Plate XXIII., B). It is 

22 M. J. Munn, “Reconnaissance of Oil and Gas Fields in Wayne and 
McCreary Counties, Kentucky,” Bull. U. S. G. S. No. 579 > I 9 t 4 > pl* 2. 

23 L. C. Glenn, communication to the writer. 


454 


A. W. LAUER. 


friable, open-textured and oolitic, qualities which are not always 
characteristic of the formation from which it comes, but which 
lend enormous storage capacity where they are present. The 
thin section reveals a cement of calcite sparingly present between 
certain of the spherulitic grains. This scarcity of cementing 
bond explains its open, friable quality. 

The rock from the McCloskey at the north end of the main oil 
field in Clark County, however, is not an oolite, but a dolomitic 
limestone. This is seen from Plate XXIII., C, to contain 
numerous good-sized openings. 

{h) Openings Induced. 

Fissures in limestones are included in the table on page 442. 
From such openings comes the oil in the fields of Tennessee and 
a part of southern Kentucky as reported by Purdue.^^ 

Under this subdivision of induced openings belong also two 
of the most important limestone oil fields of North America. 
These are the Trenton limestone fields of Ohio and Indiana, and 
the oil fields of Mexico. The high percentage of induced porosity 
to which their prolific nature is, or has been, due, consists mainly 
of two kinds: (a) For the Trenton fields, openings attributed to 
dolomitization combined with those related .to folding, fissuring, 
and jointing, with the latter in some cases enlarged by solution 
and possibly other causes, (b) For the Mexican fields, openings 
caused primarily by fissuring, possibly enlarged by solution, ac¬ 
companied by important minor openings in the form of dissolved- 
out fossil cavities. The fields will be treated in the order named. 

THE TRENTON LIMESTONE FIELDS. 

Orton^^ called attention to the fact that oil in this formation 
is found where two conditions are met, namely (a) in struc¬ 
turally favorable areas where (b) dolomitic replacement of the 

24 A. H. Purdue, state geologist of Tennessee, personal communication to 
the writer. 

25 Edward Orton, “The Trenton Limestone as a Source of Petroleum and 
Natural Gas in Ohio and Indiana,” 8t'h An. Rept. U. S. G. S., Pt. 2, 1886-1887, 
p. 662. 


PETROLOGY OF RESERVOIR ROCKS. 


455 


limestone has occurred. In Plate XXIV., a specimen of Trenton 
from Ohio is illustrated. This rock appears to be more or less 
magnesi^ft throughout, and in the large openings occur rhombo- 
hedral crystals of dolomite, which part from the walls smoothly. 
This points to their introduction subsequent to the formation of 
the cavities. These, then, are not dolomitization cavities due 
to a shrinkage of volume accompanying the replacement of lime 
by magnesia. Orton^® has pictured the microscopic structure of 
the Trenton in which openings attributed to such action are 
shown. It seems possible also that some of the openings shown 
in Plate XXIV., have resulted from the enlargement by solu¬ 
tion of original genuine dolomitization cavities. It is' also of in¬ 
terest to observe that most of the openings show a rough align¬ 
ment, or are confined to a well-defined zone, as if they were the 
result of solution by waters traveling along a seam, joint, or 
bedding plane. This fact strongly suggests the presence gen¬ 
erally throughout the limestone of enlarged open spaces along 
induced joints and cracks. 

Another feature of this specimen of Trenton is the smooth 
rounded surface of the whole mass. It has been suggested that 
abrasion by gas, or “ gas erosion,” may be the cause, and if such 
action occurred as an accompaniment of oil and gas accumulation 
in the Trenton, it would be a factor contributing toward porosity. 
Whether oil has any solvent power to create a reservoir for itself 
as it enters a formation is not known, although the writer has 
encountered the suggestion several times. 

The fact that the Trenton dolomite has yielded most of its oil 
from the upper portion, and that there is an unconformity be¬ 
tween its surface and the overlying Utica shale, suggests that a 
part of the porosity of this field may have arisen from conditions 
due to weathering in the land interval before the deposition of 
the Utica. 

THE RESERVOIR ROCKS OF THE OIL FIELDS OF MEXICO. 

The oil is found in a zone extending from near the top of the 
Tamosopo (Cretaceous) limestone into the basal portions of the 


26 op. cit., pi. 6o, opposite p. 644. 


456 


A. W. LAUER. 


overlying San Felipe (Upper Cretaceous) beds of alternating 
limestone and shale. So far as experience goes it does not occur 
far down in the 3,000 feet or more of massive limestones under 
lying the San Felipe. The upper limits are effectually set where 
the limestones of the alternating series rapidly thin and give way 
to thicker and softer, hence impervious, shales. The latter extend 
upward through an unbroken thickness of from 1,500 to 4,000 
feet, making the heavy shale cap, and appear in a good part of the 
oil belt as the surface formation. 

Where observed at the outcrop, the Tamosopo is a heavy- 
•bedded, gray, coarsely crystalline limestone (see Plate XXVI., 
A). Plate XXIII., D, is the photograph of a small piece blown 
from the Cortez Oil Corporation’s well in the hacienda of Tepe- 
tate. The vug in the upper part of the front face is a druse 
studded with tiny crystals of calcite. When first sawed in two a 
strong odor of gas issued. In the right hand piece of the en¬ 
larged view (Plate XXV., A) small specks of oil are shown. 
These issued from tiny cavities as soon as the latter were exposed 
by sawing. Others are distributed throughout the lock, as was 
discovered upon destroying one of the halves of this piece. All 
contained heavy black oil which issued under a slight pressure. 
Larger holes are present also, some on the outside of this and 
other pieces blown from the same well. A few are 'large enough 
to admit the large end of a lead pencil. So far as seen by the 
writer they range from this size down to tiny pin holes. 

The well from which this rock came is one mile north of Juan 
Casiano, the prolific camp of the Huasteca Petroleum Co. (Do- 
heny interests). It is probable therefore that the reservoir 
rock from which the immense production of Juan Casiano well 
No. 7 has come is of a similar nature. That the Tamosopo has 
this porosity over an even wider range is proved by pieces blown 
from Potrero No. 4, the famous gusher of the Mexican Eagle Oil 
Co. at their Potrero del Llano camp, twenty-five or thirty miles 
farther south. The nature of this rock is shown by the photo¬ 
graph, Plate XXVIIL, A. Whatever the extent of these open¬ 
ings, there is no question of their greatly adding to the storage 


PETROLOGY OF RESERVOIR ROCKS. 


457 


capacity, and hence as one form of effective porosity, it is of 
interest to inquire into their origin. 

So far as known, dolomitization has played no part in these 
openings. Analyses^^ of samples from Dos Bocas, Potrero del 
Llano, Alazan, and Naranjos in no case show more than 2 per 
cent, of magnesium carbonate. Salt waters from the wells of 
some of these camps have also been analyzed and show only a 
trace of magnesium salts. 

In thin section, the rock from the Cortez well is seen to be 
made up of an innumerable quantity of fossil remnants of all 
sizes, having a rather coarsely crystalline texture. They are em¬ 
bedded in an extremely fine-grained, cryptocrystalline to amor¬ 
phous calcareous cement, doubtless derived as a lime mud, or 
paste, from the constant agitation of the shells by wave action in 
a shallow sea.^® One section of this rock, ground from a por¬ 
tion having both large and small pores, reveals that the more 
coarsely crystalline calcite of the fossils is dissolved out. Various 
stages can be seen, from those in which only the central portion 
is gone, to others revealing a whole cavity conforming with the 
outline of the original fossil.These several features are por¬ 
trayed by the micro-photographs in figures B, C, D, Plate XXVI. 

Porosity, in the nature of small capillary and supercapillary 
openings, is also present in at least one locality in the San Felipe 
limestones of the alternating series overlying the Tamosopo. One 
view of the basal portion of this series, showing essentially only 
limestones, may be seen in an outcrop in the Sierra de Tamau- 
lipas about loo miles north of Tampico (Plate XXVII., A). 

Its appearance in well samples is illustrated by several pieces 
blown from Waters-Pierce^^ well No. 5 in the Topila field. Some 
of these are unaltered limestone as the one in Plate XXV., B. 
Others have been rendered cellular and porous, and are partially 

,27 Information furnished by the courtesy of E. L. DeGolyer, chief geologist 
for the Mexican Eagle Oil Co.' 

28 This explanation is suggested to the writer by Professor Chas. Schuchert. 

29 This explanation of cavities in the Tamosopo was formerly suggested to 
the writer by Mr. John M. Muir, geologist of the Corona Oil Co., Tampico, 
who thought their shape indicated such an origin. 

. 30 ]S[ow the Pierce Oil Corporation. 


458 


A. W. LAUER. 


silicified. A piece of the latter is shown in Plate XXVIL, B. 
The cellular rock is seen to have a banded arrangement, and this is 
parallel to the chert. The chert may become much thicker and is 
widely noted in the drillings. Sometimes it is present in nodular 
lenses as indicated by the piece in Plate XXVIL, C. 

The foregoing rocks are of interest when studied in thin sec¬ 
tion, first for their composition, and second for the evidence they 
furnish leading to the explanation of the cellular, banded porosity. 
They are all made up of myriads of foraminifera, mostly present 
in small round forms, but in one section consist chiefly of elon¬ 
gated lenticular shapes. These are bedded in a limy, argillaceous 
matrix. Often they are rather evenly distributed, but frequently 
are gathered in clusters and aggregates covering a considerable 
area. In some of the rocks they are of crystalline calcite—their 
normal composition—in others they have been largely replaced 
by opaline silica. In numerous cases mineral matter is lacking, 
and only a round or elongate hole appears in the section. In a 
few cases the holes were seen to be partially opened foraminiferal 
cavities, showing some of the peripheral circular outline remain¬ 
ing. A number of the larger holes have hour-glass shapes. 
Others are large and irregular, but with semi-circular parts in 
their boundaries. The holes are generally in a parallel arrange¬ 
ment similar to the foraminifera of some of the sections. 

These facts rather clearly indicate the explanation for this 
banded, cellular porosity, namely a solution of the lime tests of 
the foraminifera with only partial replacement by silica. The 
large holes, which are often many times the size of the average 
foraminifera, may be explained by the solution of the local, densely 
packed portions of the rock. This is strongly suggested by their 
lobed outlines. The banded tubular openings correspond to the 
banded arrangement of the fossils. 

A parallel arrangement of foraminifera was observed also in 
specimens from Mexican Oil Co. well No. 3, Topila field, and 
for Corona well No. i, Panuco field. But these rocks show no 
porosity, and the foraminifera have their normal composition of 
calcite. 

With the explanation of these fossil pores only a part of the 



PETROLOGY OF RESERVOIR ROCKS. 


459 



Fig. 17. Map of a part of the southern oil fields, Mexico. After Huntley. 














460 


A. W. LAUER. 


effective porosity of the Mexican oil reservoirs has been men¬ 
tioned. Reference to the table of limestone openings on page 
452, coupled with the faulted structures found in Mexico, at once 
suggest important openings induced by faulting and fracturing. 
The history of wells and their location bear this out. In so far 
as the several systems of fractures exist which are indicated on 
Huntley’s map^^ (see Fig. 17) they add to the strength of the 
suggestion by permitting the inference that complex stresses, per¬ 
haps torsional, aided in breaking up these limestones and jointing 
and fracturing the harder San Felipe shales. That these frac¬ 
tures have been enlarged by solution is also not improbable, nor 
is it to be overlooked that movement along a sinuous fracture 
surface, as suggested by Lindgren,^^ may cause openings between 
alternate touching points. It is not quite clear to the writer just 
how much Huntley^^ wishes to include in- the term “ water chan¬ 
neling,” but it is presumed he means joints, bedding partings and 
fractures enlarged by solution in the sense indicated in the first 
part of the preceding sentence. 

Some geologists express the opinion that considerable dis¬ 
turbance of the oil-bearing sediments of Mexico has resulted 
from causes associated with their intrusion by the many plugs, 
dikes, and sills of basalt which occur there. In so far as this is 
true it affords an additional explanation for induced openings 
which may contribute to pore space. 

In conclusion it may be said that a study of the Mexican sam¬ 
ples forcibly suggests an entrance of the oil into the present reser¬ 
voir through the coarser openings, with subsequent impregnation 
of the surrounding finer pores. It appears that it has soaked into 
the rock bounding the trunk channels. 

While these rocks are limestones, this view may just as con¬ 
sistently be applied to close textured sandstones, and to some 
shales, where the principal storage space seems to be in super- 
capillary induced openings. This view is in contradistinction to 

21 L. G. Huntley, “The Mexican Oil Fields,” Trans. A. I. M. E., Vol. 52, 
1915, p. 310, Fig. 6. 

32 Waldemar Lindgren, “Mineral Deposits,” first edition, 1914, p. 139. 

33 Op. cit., pp. 305, 313, 314. 


OPENINGS IN SHALES 


■i'TJr 






Super-capillary (>.508-.254 Mm.) ' Capillary ( <.508-.254 Mm.) 


(1.) Drying cracks. 
( 2 .) - 


ORIGINAL 

(1.) Drying cracks. 
(2.) Rock pores. 

INDUCED 


(1.) Due to folding (fissures, bedding 
partings, synclinal slumping). 

(2.) Compressional brecciation and con¬ 
jugate fissures. 

(2). Eissuring and ruptur’ng apart from 
folding (joints, fissures by torsional 
stress). 

(4). Effects of former surface weatherl 
ing below unconformities. 


(1 ) Due to folding (fis¬ 
sures, bedding part¬ 
ings, synclinal slump¬ 
ing) . 

(2.) Compressional breccia¬ 
tion and conjugate 
fissures. 

(3 ) Fissuring and ruptur¬ 
ing apart from fold¬ 
ing (joints and fis- 
ures by tors’onal 

stress). 

(4.) Effects of former sur¬ 
face weathering be¬ 
low unconformities. 
















PETROLOGY OF RESERVOIR ROCKS. 


461 


what seems to have been commonly held, that the oil is present 
mainly in interstitial pores with merely an exit through larger 
openings. It implies a working into the country rock throughout 
geologic time, rather than the reverse process of working outward 
from a position imprisoned in interstitial rock pores. The latter 
process probably obtains during movement from the beds of 
origin, while the former is characteristic of movement into the 
rocks of accumulation. 

Shales. 
w. ^ ^nal. 

Super-capillary (> .508— .254 Mm.). 

( 2 )- 

Induced. 

(1) Due to folding (fissures, bedding partings, 

synclinal slumping). 

(2) Compressional brecciation and conjugate 

fissures. 

(3) Fissuring and rupturing apart from fold¬ 

ing (joints, fissures by torsional stress). 

(4) Effects of former surface weathering be¬ 

low unconformities. 


Capillary (< .508 — .‘’54 Mm.). 


(1) Drying cracks. 

(2) Rock pores. 


DISCUSSION OF TABLE. 

Shales become effective oil reservoirs only if hard enough to 
offer resistance to deformation, when they yield by folding and 

4 

fracturing. In this sense only can the various classes of induced 
openings listed in the foregoing table develop. When very soft, 
on the contrary, deformation will simply mold the shales, leaving 
them close and impervious. Because of frequent tendency to de¬ 
velop thin beds, or closely spaced laminae, many joints develop 
when these thin beds suffer deformation. It is probable that some 
joints also originate as drying cracks. 

Whatever the several causes are, a considerable induced poros¬ 
ity may be developed in such hard shales, which can be effective 
for the storage of petroleum. 


462 


A. W. LAUER. 


ILLUSTRATIVE TYPES. 

The harder shales interbedded with the limestones to form the 
alternating, or San Felipe, series have been mentioned in the dis¬ 
cussion of the Mexican fields. These are notably resistant where 
observed in the beds outcropping in and near the town of Valles, 
state of San Luis Potosi, 70 or 75 miles west of Tampico. That 
these shales form a part of the oil reservoir rocks is proved by 
many wells never reaching the top of the Tamosopo limestone, 
and by their being ejected from newly drilled gushers. Such 
specimens are illustrated in Plate XXV., C, and Plate XXVIIL, 
B and C. All of these shales are dense, close, hard, and ap¬ 
parently dry. They are also very limy. Moistened with water, 
capillary action is noticeably slow. Hence they appear to fail 
entirely in taking up the more viscous, heavy, Mexican oil. The 
low surface tension of oils is also probably instrumental in keep¬ 
ing it out of these shales. 

As further illustrating the occurrence of oil, wholly or in part, 
in broken shales on this continent, may be mentioned the oil fields 
of Florence, Colorado, Katalla and Cook Inlet, Alaska, and the 
Santa Maria field, California. 

CONCLUSION. 

A review of the preceding evidence reveals the enormous im¬ 
portance of induced porosity in oil accumulation. This applies to 
reservoir rocks of all three types, sandstones, limestones, and 
shales. It entirely overshadows the original porosity of an inter¬ 
stitial nature except in the case of loosely consolidated sands. 
This is true not alone because any one type of induced openings 
may afford large storage, but also because there are so many 
causes contributing toward induced porosity. 

Thus there are openings first, due to folding, which manifest 
themselves as fissures and bedding partings; second, resulting 
from compressive stresses, producing brecciation, and sometimes 
complex and conjugate fissure systems; third, dolomitization 
cavities in altered limestones; fourth, fissures and rupturing of 


PETROLOGY OF RESERVOIR ROCKS. 


463 


rock beds from causes other than folding, from which result 
numerous joints and torsional breaks; fifth, important enlarge¬ 
ment by solution of all the other types of porosity; sixth, cavities 
arising from the solution of fossil forms present in the rocks; 
and finally, a source of openings not before emphasized in the 
literature, if referred to at all, namely the effects of weathering 
agencies, percolating waters, chemical activity, and organic de¬ 
struction which must have acted on every formation exposed to 
atmospheric agencies such as those now marked by unconformities 
at their upper terminations. 

All discussions of porosity heretofore published have been of 
too limited a scope, and have erred in not sufficiently recognizing 
the part played by openings of an induced nature. They have 
furthermore gone to the extreme in this limited discussion of 
original porosity by attempting to find accurate percentages of 
voids based on the absorptive properties of oil rocks. It has been 
shown how wide of the mark such computations are, and how un¬ 
reliable even in the case of loose sands where interstitial porosity 
is the dominant variety. A better method of estimating future 
production is the production curve, which is based on actual yields 
of oil and therefore must include the effect of every factor enter¬ 
ing into production for the field tested. 

Porosity tests have all gone back to the original King and 
Schlichter method of finding how much space is contained be¬ 
tween sand grains, and this general scheme of computation has 
been followed and referred to in foreign journals. Thus it has 
happened that a porosity factor of often minor consideration has 
been dealt with alone, to the exclusion and disregard of all the 
other, often much more potent factors. 

Recognizing porosity then from this broader viewpoint, it is 
easy to come to a conception of underground water circulation 
controlled by hydrostatic laws. If this be granted an important 
sidelight on oil accumulation is at hand, and the widespread in¬ 
duced openings serve not only a freer water movement, but also 
to reestablish the anticlinal theory. When, in addition to this, 
it be remembered that the authors of other theories have not 


464 


A. W. LAUER. 


wholly denied the important value of anticlinal accumulation, and 
seek to harmonize their new theories with it, it seems fairly con¬ 
clusive that the conservative statement of the anticlinal theory 
quoted in the beginning of this paper should stand as established. 

ACKNOWLEDGMENTS. 

The writer is deeply indebted to the following sources for sta¬ 
tistical data, rock specimens, and personal help in the preparation 
of this paper: 

To his former colleague, Mr. John M. Muir, geologist for the 
Compania de Petroleo la Corona, Tampico, Mexico, and to the 
manager, Mr. C. Van Geytenbeek, and other representatives of 
that company, he is especially grateful for a large and represen¬ 
tative suite of specimens from the reservoir rocks and associated 
strata in the Mexican oil fields. Mr. Muir has also been most 
kind in gathering statistical data and answering questions which 
came up in the course of writing. 

To Mr. R. A. Conkling, head geologist of the Roxana Pe¬ 
troleum Co., and to Mr. James H. Gardner, consulting oil geolo¬ 
gist, both of Tulsa, Oklahoma, sincere thanks are due for sam¬ 
ples of sands from the oil horizons of the Mid Continent field. 

Samples from Illinois were kindly furnished by the State 
Geological Survey; from Ohio by Mr. J. A. Bownocker, state 
geologist; and from Kentucky by Dr. L. C. Glenn, of Vanderbilt 
University, Nashville, Tenn. From the California fields, a suite 
representing both surface rocks and the oil reservoirs were sup¬ 
plied by Mr. W. W. Orcutt, geologist for the Union Oil Co., Los 
Angeles. 

Some production data from these fields were added by Mr. 
Leon J. Pepperburg, consulting geologist, San Francisco; and 
data of occurrence of the oils in Tennessee and Kentucky are at 
hand from Mr. A. H. Purdue, director of the State Geological 
Survey of Tennessee. 

Valuable data were received from the reports of the United 
States Geological Survey and from many other books and jour- 


PETROLOGY OF RESERVOIR ROCKS. 


465 


nals treating on petroleum. For these and to such other sources 
not herein specified, acknowledgment is cited in the footnotes. 

Finally, it is the writer’s great pleasure to mention the inval¬ 
uable suggestions received, not only as to handling material, but 
also in criticism of manuscript, from the following professors of 
geology in Yale University: J. D. Irving, L. V. Pirsson, Charles 
Schuchert, Joseph Barrell, Wm. E. Ford, and Alan M. Bateman. 


466 


A. IV. LAUER. 


EXPLANATION OF PLATE XXIL 

Fig. a. Hoing sand from Colmar field, McDonough county, Illinois. Nat¬ 
ural size. 

Fig. B. Sandstone from the producing horizon of a well in the Cushing 
field, Oklahoma. Natural size. 

Fig. C. Grains of sand from the sandstone in Fig. B. Magnification 16 
diameters. 

Fig. D. Very porous oil-bearing limestone from Wayne county, Kentucky. 
Enlarged 2 diameters. After Munn. 


Plate XXII 


Economic Geology. Vol. XII 



A 


B 




D 












Plate XXIII 


EcoNOvic Geology. Vol. XII 




A 



B 



C 



D 




















PETROLOGY OF RESERVOIR ROCKS. 


467 


EXPLANATION OF PLATE XXIII. 

Fig. a. Corniferous limestone from the Irvine, Kentucky, field. Left to 
right: First, porous specimens from outcrop in railroad cut, town of Irv'ine; 
second, less porous portion of same outcrop; third, piece of “tight” hme- 
stone shot from a well on Station Camp Creek, six miles S.S.E. of Irvine. 
Natural size. 

Fig. B. McCloskey “ sand ” from a well in Lawrence count}-, Illinois. 
Right, piece sawn in two. Left, polished face of left hand portion. Natural 
size. 

Fig. C. Specimen of dolomite from McCloskey “ sand,” Illinois. Left 
polished face of one portion. Natural size. 

Fig. D. a piece of Tamosopo limestone blown from well of Cortez Oil 
Corporation, hacienda of Tepetate. Natural size. 


468 


A. W. LAUER. 


EXPLANATION OF PLATE XXIV. 

Fig. a. a piece of Trenton limestone shot from a well near Findlay, 
Ohio, at a depth of 1,425 feet. Four fifths natural size. 

Fig. B. Same piece as in preceding figure seen from reverse side. Four 
fifths natural size. 


Plate XXIV 


Economic^Geology. Vol. XII 




B 

















Plate XXV. 


Economic Geology. Vol. XII. 




















PETROLOGY OF RESERVOIR ROCKS. 


469 


EXPLANATION OF PLATE XXV. 

Fig. a. Enlarged view of pieces in Plate XXIII., Fig. D. 

Fig. B. Specimen of San Felipe limestone blown from Waters-Pierce well 
No. 5, Topila field, Mexico. Natural size. 

Fig. C. Specimen of San Felipe shale from well No..2 of the Tal Vez 
Oil Co., Panuco field. Left hand sectional view reveals a complete lack of 
penetration by oil, although specimen was immersed in it. 


470 


A. I//. LAUER. 


EXPLANATION OF PLATE XXVL 

Fig. a. Tamosopo limestone in an upthrust block, Micos Canyon, on the 
railway from Tampico to San Luis Potosi. 

Fig. B. Section of Tamosopo showing portion of its fossil structure. 
X 20 diameters. 

Fig. C. Cavities caused by fossils dissolved out of Tamosopo limestone. 
X 20 diameters. 

Fig. D. Same as above. Note fragments still remaining, undissolved, in 
some holes. X 20 diameters. 


Plate XXVI. 


Economic Geology. Vol. XII. 



B 


D 






Plate XXVII 


Economic Geology. Vol. XII 




B 



C 














PETROLOGY OF RESERVOIR ROCKS. 


471 


EXPLANATION OF PLATE XXVII. 

Fig. a, San Felipe limestone outcrop in the Sierra de Tamaulipas, 100 
miles north of Tampico. 

Fig. B. San Felipe limestone blown from Waters-Pierce well No. 5 , To- 
pila field, Mexico. Light' portion (left hand view) chert. Dark is oil- 
soaked cellular rock. Natural size. 

Fig. C. Cherty nodule blown from Waters-Pierce well No. 5. Left hand 
sectional view, white is cherty material and very hard; black is soft oil- 
soaked rock enclosing it above and below. 


472 


A. W. LAUER. . 


EXPLANATION OF PLATE XXVIII. 

Fig. a. Specimen of Tamosopo limestone from well No. 4 , Potrero del 
Llano. Copy of a photograph presented to the writer. 

Fig. B. Specimen of shale blown from Harmon gusher, Panuco field. 
This is much harder than that shown in PI. XXV., Fig. C. Sectional view 
shows total absence of penetration by oil. 

Fig. C. Another specimen of shale from Harmon well. Where chipped 
below upper right-hand corner, the white spot shows clean interior. 


Plate XXVIII 


Economic Geology. Vol. XII 







































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